Last update:
09/22/2008
The spiral-wound element is an industry standard because of its high membrane area packing density, low cost of manufacture, and extensive installation base worldwide. Plastic mesh spacers create a feed-channel between facing membrane leafs and promote turbulent flow, which aids to reduce solute concentration polarization and related scaling and fouling phenomena. Computational and experimental studies have demonstrated complex hydrodynamics that vary depending on the spacer geometry, thickness, and membrane packing density. Analyses of fouling deposits on RO membranes, removed from spiral wound elements, have revealed that channel spacers strongly influence foulant deposition patterns. For example, the presence of ill-designed feed spacers can create stagnant zones with elevated concentration polarization. These stagnant regions adversely affect membrane performance by promoting fouling and scaling locally, thus reducing flux, limiting water recovery, and lowering permeate quality.
Previous studies on membrane hydrodynamics and concentration polarization
have resorted to simplified analytical models, or numerical models that are
fundamentally inaccurate due to uncoupling of the equations of motion, which
describe channel hydrodynamics, from the mass conservation equations, which
describe rejected solute transport. As a consequence, only limited
information is known regarding the complex hydrodynamic and concentration
polarization fields that develop around spiral wound element feed spacers.
The highly oversimplified past models were incapable of handling complex
geometries and flow patterns in which flow instabilities may arise. However,
with recent development in finite-element simulations of fluid flow and
solute transport, it is now feasible to develop accurate models of the
hydrodynamic and concentration fields in spiral-wound membrane elements
employing any spacer geometry. Accordingly, a comprehensive computational
approach will be undertaken to evaluate and optimize module hydrodynamics.
A comprehensive finite-element model will be developed to describe spiral
wound element hydrodynamics and concentration polarization for conventional
and novel spacer geometries. The multi-physics model will consist of the
fully-coupled equations of motion and species convection-diffusion equations
including phase equilibria relationships for multi-ion systems. Numerical
simulations will be carried out for a range of conditions representative of
brackish water desalination. The impact of spacer design (i.e., size,
geometry, orientation) and element packing density will be analyzed to aid
development and design of new spacer geometries designed to promote intense
vortex shedding and eliminate stagnant zones. Results of these simulations
will provide detailed three-dimensional views of concentration polarization,
thereby enabling complete optimization of spacer geometry and arrangement so
as to achieve maximum reduction in concentration polarization, scaling, and
fouling.
The above model will be coupled with models of mineral salt crystallization
(scaling) and colloidal deposition (fouling) already developed through other
supporting projects of the UCLA WaTeR Technology Research Center to predict
scaling and fouling under varying operating conditions, spacer geometry, and
membrane properties. Detailed numerical simulations will enable further
refinement and selection of a spacer and module design best suited for
brackish water desalination. Prototype spacers will be constructed for
experimental evaluation via laboratory scale membrane performance tests.
Fundamental areas: fluid mechanics, mass transfer, surface science, membrane science, numerical methods
Finite element simulation results illustrating the hydrodynamics (streamlines) in a membrane flow channel with spacers arranged in two different geometries. High local concentration polarization can occur in the recirculation regions, at the downstream side of the spacers, resulting in increased fouling.
Modeling approach for the development of guidelines for reduction of membrane fouling by optimization of module hydrodynamics.
